Long wave infrared transparent window and coating materials

ABSTRACT

Systems and method for forming a nanocomposite material. One example of a nanocomposite material includes a first sulfur-based nanoparticle material defining a first nanophase and a second sulfur-based nanoparticle material defining a second nanophase, wherein the nanocomposite material is at least partially long-wave infrared (LWIR) transmitting, and the first nanophase and the second nanophase are co-dispersed to form interpenetrating networks with one another and each has a grain structure that is distinct from one another.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of, and claims priority to and thebenefits under 35 U.S.C. §§ 120 and 121, of co-pending U.S. applicationSer. No. 14/478,285 filed Sep. 5, 2014 and titled “LONG WAVE INFRAREDTRANSPARENT WINDOW AND COATING MATERIALS,” the entirety of which isincorporated by reference herein for all purposes.

BACKGROUND

Optical imaging systems generally include one or more externally mountedoptical elements which shield the remainder of the imaging system froman external environment. For example, with infrared (IR) airborneimaging systems, an IR transparent optical element such as a window ordome is generally mounted on the airborne system to isolate theremainder of the IR imaging system from exposure to humid, corrosive,and/or abrasive environments. Prolonged exposure to these environmentsgenerally degrades the optical and physical characteristics of thematerial of the external optical element. In certain instances the mostsevere environmental exposure encountered by such external opticalelements is high velocity water droplet impact that occurs when anairborne system is flown through a rain field. In addition, externaloptical elements are harmed by dust particles, such as sand, which mayoccur in desert environments.

In general, exposure to water droplet impact is referred to as rainerosion. During flight through a rain field, water droplets from a rainfield impinge upon the surface of the external element producingsubsurface fractures even at subsonic velocities. For very brittlematerials, these subsurface fractures are initiated at pre-existingmicroflaws lying near or at the surface of the optical element. Rainerosion damage to such optical elements occurs prior to any significantremoval of material. The mere propagation of these pre-existingmicroflaws is sufficient to damage the optical element. In particular,these microflaws are propagated through the optical element by thetensile component of a surface stress wave created at the time of impactwith the water droplet. Once formed, the continued propagation of asubsurface fracture through the optical element will often produce largecracks in the optical element. In the region of the cracks, scatteringand refraction of incident IR energy will occur that ends up producingincreased internal reflections and IR energy losses. With a significantnumber of such cracks, the transmissivity of the optical element isseverely reduced. Furthermore, as cracks propagate through the opticalelement, catastrophic failure of the element may occur. When the opticalelement shatters or breaks, the remaining optical elements of the IRimaging system are exposed to the external environment, resulting inpotential catastrophic damage to the imaging system. Similar types ofproblems may also be caused by abrasion from sand particles. Evenfurther, for airborne systems such as aircraft or missiles, damage tothe window or dome may cause loss of control of the airborne system,which may be catastrophic.

Non-limiting examples of materials which offer the best mechanicaldurability and optical performance for infrared imaging systems, such aslong wavelength infrared (LWIR) energy in the 8.0 micron to 12.0 microninfrared band, include zinc sulfide (ZnS), zinc selenide (ZnSe),germanium (Ge), gallium arsenide (GaAs), gallium phosphide (GaP),mercury cadmium telluride (HgCdTe), and cadmium telluride (CdTe).However, these materials are relatively brittle and have a relativelylow resistance to damage, particularly damage sustained during highvelocity impact from water droplets and dust particles, such as sand.For example, ZnS and ZnSe are relatively soft and lack durability whenthey are exposed to severe environmental conditions. To furthercomplicate matters, coating materials that are hard may also be moreabsorbing, in particular at LWIR wavelengths. In addition, rain enhancedprotective (REP) ZnS coatings deposited using radio frequency (RF)magnetron sputtering result in highly compressive stressed films thattend to delaminate from the base material during impact.

Optical energy incident upon a surface of an optical element results inreflection of the energy at the surface if the index of refraction ofthe material comprising the optical element is significantly differentthan the index of refraction of the medium from which the energyoriginates. Generally, for airborne systems, the originating medium isair having an index of refraction of about one. Accordingly, it isdesired to provide optical elements and coatings using materials ofappropriate refractive index to reduce losses attributed to reflection.

SUMMARY OF THE INVENTION

Aspects and embodiments relate generally to durable long-wave infrared(LWIR) transmitting materials, and more particularly, to multi-phasenanocomposite materials for constructing and shielding optical elements.Certain aspects and embodiments are directed to a combination of two ormore LWIR transmitting materials that are mutually insoluble to formnanocomposite materials that exhibit superior resistance to abrasion anderosion caused by high velocity liquid and solid particles, includingrain and sand.

According to one embodiment, a nanocomposite material comprises a firstsulfur-based nanoparticle material defining a first nanophase, and asecond sulfur-based nanoparticle material defining a second nanophase,wherein the nanocomposite material is at least partially long-waveinfrared (LWIR) transmitting, and the first nanophase and the secondnanophase are co-dispersed to form interpenetrating networks with oneanother and each has a grain structure that is distinct from oneanother.

In one example, the first and the second sulfur-based nanoparticlematerials are mutually insoluble, and at least one of the first and thesecond sulfur-based nanoparticle material is insoluble in water. In oneexample, the first sulfur-based nanoparticle material and the secondsulfur-based nanoparticle material form a layer of nanocompositematerial having a hardness value that is greater than a hardness valueof either the first or the second sulfur-based nanoparticle materials.In another example, the layer of nanocomposite material forms an opticalelement having a thickness of from about 1 mm to about 25 mm. In anotherexample, the layer of nanocomposite material forms a coating having athickness of less than about 50 microns. In another example, the coatingof nanocomposite material coats a surface of an optical element formedfrom the nanocomposite material. In one example, the first sulfur-basednanoparticle material is zinc sulfide (ZnS). In one example, the secondsulfur-based nanoparticle material is Calcium Lanthanum Sulfide (CLS).In one example, the second sulfur-based nanoparticle material is yttriumsulfide (Y₂S₃). In one example, the volume ratio of the firstsulfur-based nanoparticle material to the second sulfur-basednanoparticle material is in a range of about 10:90 to about 90:10.

According to another embodiment, a method of forming a nanocompositematerial comprises combining a plurality of nanoparticles of a firstsulfur-based material with a plurality of nanoparticles of a secondsulfur-based material such that the plurality of nanoparticles of thefirst sulfur-based material define a first nanophase, and the pluralityof nanoparticles of the second sulfur-based material define a secondnanophase, and using the combination of the plurality of nanoparticlesof the first sulfur-based material and the plurality of nanoparticles ofthe second sulfur-based material to produce a layer of the nanocompositematerial, wherein the layer of nanocomposite material is at leastpartially LWIR transmitting, and the first nanophase and the secondnanophase are co-dispersed to form interpenetrating networks with oneanother and each has a grain structure that is distinct from oneanother.

In one example, the layer of nanocomposite material has a hardness valuethat is greater than a hardness value of either the hardness value ofthe nanoparticles of the plurality of the first sulfur-based materialand the hardness value of the nanoparticles of the plurality of thesecond sulfur-based material. In one example, the plurality ofnanoparticles of the first and the second sulfur-based materials aremutually insoluble and at least one of the plurality of nanoparticles ofthe first and the second sulfur-based material is insoluble in water. Inone example, combining includes uniformly mixing and isostaticallypressing the plurality of nanoparticles of the first and the secondsulfur-based materials such that the layer of nanocomposite materialforms an optical element having a thickness of from about 1 mm to about25 mm. In one example, combining includes sputtering the plurality ofnanoparticles of the first sulfur-based material with the plurality ofnanoparticles of the second sulfur-based material such that the layer ofnanocomposite material forms a coating having a thickness of less thanabout 50 microns. In another example, combining to form the coatingincludes coating a surface of an optical element formed from thenanocomposite material. In another example, the plurality ofnanoparticles of the first sulfur-based material are ZnS, and theplurality of nanoparticles of the second sulfur-based material are CLSor Y₂S₃. In one example, the volume ratio of the plurality ofnanoparticles of the first sulfur-based material to the plurality ofnanoparticles of the second sulfur-based material is in a range of about10:90 to about 90:10.

According to another embodiment, an optical element formed of ananocomposite material comprises a first sulfur-based nanoparticlematerial defining a first nanophase, and a second sulfur-basednanoparticle material defining a second nanophase, wherein thenanocomposite material is at least partially long-wave infrared (LWIR)transmitting, and the first nanophase and the second nanophase areco-dispersed to form interpenetrating networks with one another and eachhas a grain structure that is distinct from one another.

Still other aspects, embodiments, and advantages of these exampleaspects and embodiments, are discussed in detail below. Moreover, it isto be understood that both the foregoing information and the followingdetailed description are merely illustrative examples of various aspectsand embodiments, and are intended to provide an overview or frameworkfor understanding the nature and character of the claimed aspects andembodiments. Embodiments disclosed herein may be combined with otherembodiments, and references to “an embodiment,” “an example,” “someembodiments,” “some examples,” “an alternate embodiment,” “variousembodiments,” “one embodiment,” “at least one embodiment,” “this andother embodiments” or the like are not necessarily mutually exclusiveand are intended to indicate that a particular feature, structure, orcharacteristic described may be included in at least one embodiment. Theappearances of such terms herein are not necessarily all referring tothe same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide an illustration anda further understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of any particular embodiment. Thedrawings, together with the remainder of the specification, serve toexplain principles and operations of the described and claimed aspectsand embodiments. In the figures, each identical or nearly identicalcomponent that is illustrated in various figures is represented by alike numeral. For purposes of clarity, not every component may belabeled in every figure. In the figures:

FIGS. 1A and 1B are side views of optical elements according to aspectsof the invention;

FIG. 2 is a phase diagram of a ZnS—Sm₂S₃ system according to aspects ofthe invention;

FIG. 3 is a table of candidate nanocomposite components according toaspects of the invention;

FIG. 4 is a graph showing the relationship between predicted hardnessand grain size according to aspects of the invention;

FIG. 5 is a block flow diagram of a processing method according toaspects of the invention;

FIG. 6 is a diagram of deposition apparatus according to aspects of theinvention;

FIG. 7 features two scanning electron microscope (SEM) images of ananocomposite material according to aspects of the invention;

FIG. 8 features a SEM image and two image analysis graphs of ananocomposite material according to aspects of the invention;

FIG. 9 is an X-ray diffraction (XRD) spectrum of a nanocompositematerial according to aspects of the invention;

FIG. 10 is a plot of percent transmission versus wavelength for a singlelayer of nanocomposite material according to aspects of the invention;and

FIG. 11 is a plot of percent transmission versus wavelength for severalthicknesses of single layer nanocomposite coating material according toaspects of the invention.

DETAILED DESCRIPTION

By way of introduction, aspects and embodiments relate to systems andmethods for providing nanocomposite materials possessing LWIRtransmitting capabilities and superior physical properties renderingthem capable of withstanding exposure to severe environmentalconditions. In some embodiments, the nanocomposite material is used toconstruct optical elements, such as windows. In other embodiments, thenanocomposite material is used as a nanocomposite coating that issuitable for protecting optical elements from environmental exposure. Asused herein, the term “nanocomposite” refers to a multi-phase compositematerial comprising a mixture of two or more nanoparticle materials. Asdiscussed further below, according to various embodiments, the two ormore nanoparticle materials are mutually insoluble and therefore form amulti-phase structure that exhibits superior physical properties whencompared to the physical properties of the individual parent materials.The nanocomposite materials disclosed herein show superior resistance tosand and rain erosion, thereby extending the lifetime of LWIR windowsand domes.

The aspects disclosed herein in accordance with the present invention,are not limited in their application to the details of construction andthe arrangement of components set forth in the following description orillustrated in the accompanying drawings. These aspects are capable ofassuming other embodiments and of being practiced or of being carriedout in various ways. Examples of specific implementations are providedherein for illustrative purposes only and are not intended to belimiting. In particular, acts, components, elements, and featuresdiscussed in connection with any one or more embodiments are notintended to be excluded from a similar role in any other embodiments.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. Any references toexamples, embodiments, components, elements or acts of the systems andmethods herein referred to in the singular may also embrace embodimentsincluding a plurality, and any references in plural to any embodiment,component, element or act herein may also embrace embodiments includingonly a singularity. References in the singular or plural form are notintended to limit the presently disclosed systems or methods, theircomponents, acts, or elements. The use herein of “including,”“comprising,” “having,” “containing,” “involving,” and variationsthereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. References to “or” maybe construed as inclusive so that any terms described using “or” mayindicate any of a single, more than one, and all of the described terms.In addition, in the event of inconsistent usages of terms between thisdocument and documents incorporated herein by reference, the term usagein the incorporated reference is supplementary to that of this document;for irreconcilable inconsistencies, the term usage in this documentcontrols. Moreover, titles or subtitles may be used in the specificationfor the convenience of a reader, which shall have no influence on thescope of the present invention.

Optical Element

According to at least one embodiment, an optical element is providedthat is constructed from one or more nanocomposite materials. Referringto FIG. 1A, an optical element, generally indicated at 100, is shownthat includes a dome 102 suitable for use on an exterior surface of anaircraft, satellite, or missile. The dome 102 comprises a nanocompositematerial 104 as discussed further below. Although the optical element inFIG. 1A features a dome, it is appreciated that other types of opticalelements having different sizes and shapes, such as plates, windows,lenses, etc., may alternately be substituted for the dome 102. Dependingon the specific application and the type of materials used, thenanocomposite material 104 used to construct the optical element 100 mayhave a thickness in a range of about 1 mm to about 25 mm. According toat least one embodiment, the nanocomposite may have a grain size ofabout 1 micron to about 10 microns.

Coating

According to another embodiment, a nanocomposite coating is providedthat may be used to cover at least a portion of an optical element, suchas a window, dome, lens, etc. used for aerospace or industrialapplications. Referring to FIG. 1B, an optical element, generallyindicated at 101, is shown that includes a substrate 103, and at leastone layer of a nanocomposite coating material 105, as discussed furtherbelow. The substrate 103 shown in FIG. 1B is planar in shape, but othershapes, such as the dome 102 in FIG. 1A are also possible, depending onthe specific application. A single layer of the nanocomposite coating105 may be used, or in some instances, multiple coatings may be used.Further, the thickness of the nanocomposite coating material varies withthe type of specific application. According to some embodiments, thethickness of the nanocomposite coating may be less than about 50microns. According to other embodiments, the thickness of thenanocomposite coating may be in a range of about 15 microns to about 25microns. In accordance with other embodiments, the thickness of thenanocomposite coating may be less than 1 micron. The nanocompositecoating may be any thickness that is suitable for the purposes ofperforming a protective function as described in the methods and systemsdisclosed herein.

According to a further embodiment, the substrate 103 of FIG. 1B maycomprise a nanocomposite material as disclosed herein. For example, thesubstrate 103 may comprise a first nanocomposite material and thenanocomposite coating material 105 may comprise a second nanocompositematerial. Even further, both the substrate 103 and the coating 105 maycomprise the same nanocomposite material. In certain instances, this maybe advantageous, since the substrate and coating have matchingrefractive indices and material properties, thereby enhancing theirability to bond with one another and reduce light scatter at theirinterface.

Properties of Candidate Nanocomposite Materials

According to one or more embodiments, a nanocomposite material isprovided that comprises two or more nanoparticle materials. As usedherein, the term “nanoparticle” refers to particles having a diameter ofless than 500 microns. The nanoparticle may be of any shape, includingspherical shapes having crystalline structures, such as those listed inFIG. 3, as well as other shapes, such as platelets, whiskers, fibers,etc. The nanoparticles may be a compound comprising one or moreelements. For example, at least one element of the nanoparticle materialmay be sulfur (S), and thus may be referred to herein as a sulfur-basednanoparticle. Further, nanocomposite materials that include at least onesulfur-based compound may be referred to herein as a sulfur-basednanocomposite. As used herein, the term “sulfur-based” also refers tosulfides or sulfates, depending on the chemical composition of thenanoparticle or nanocomposite.

According to a further aspect, the nanoparticle materials are at leastpartially LWIR transmitting. As used herein, LWIR refers to long-waveinfrared radiation in the range of 8.0 microns to 12.0 microns (λ=8-12μm).

In accordance with some embodiments, the nanoparticle materials may havea refractive index that is compatible with a substrate material, such asin applications where the nanocomposite material is used to coat anoptical element. Further, in embodiments where the nanocompositematerial comprises one or more nanoparticle materials, each nanoparticlematerial used in the nanocomposite may have a refractive index that issimilar in value to the other nanoparticle materials used in thenanocomposite. For example, the nanoparticle materials used in ananocomposite coating or optical element may possess refractive indicesthat are very close in value to one another. This ensures that thematerials are optically compatible, thereby minimizing any loss ofoptical information transmitted through the coating and/or opticalelement.

According to a further aspect, when the nanoparticle materials areincorporated into a nanocomposite material, the resulting hardness ofthe nanocomposite material is greater than the hardness of either of thestarting nanoparticle materials, thereby resulting in enhancedresistance to abrasive and environmental forces. As mentioned above, atleast one of the nanoparticles may be sulfur-based. In general, sulfidesare harder than selenides and tellurides. Thus, certain aspectsdisclosed herein are directed toward sulfide-based materials.

According to some embodiments, the two or more nanoparticle materialscomprising the nanocomposite are mutually insoluble. As used herein, theterm “mutually insoluble” refers to a combination of two or morenanoparticle materials that do not form a single phase of a solid. Forexample, according to one embodiment, a two-phase grain structureremains distinct, even after full processing is achieved such that thereis distinct phase separation between the two constituents. According tocertain aspects, the two phases are generated simultaneously (based onthe thermodynamics of the system during processing) and forminterpenetrating networks with one another. Each phase of themulti-phase structure may be a nanophase, otherwise referred to hereinas a nanoscale phase. As used herein, the terms “nanophase” or“nanoscale phase” refer to a solid phase where at least one dimension issubstantially less than 1 micron. For example, each phase of themulti-phase structure is nanoscale in size, i.e., no more than tens ofnanometers. Without being bound by theory, it is hypothesized that themulti-phase structure of the nanocomposite material provides physicalproperties related to hardness and durability. According to someaspects, certain processing conditions may be used to ensure thatnanoscale phases are preserved during processing. For example, lowdeposition temperatures may be used during processing, thereby allowingone or more of the nanophases to remain small. Specific processingconditions are dependent on the type of material being used and thespecific targeted application, but in general terms, temperatures,pressures, and processing times are kept low enough to prevent rapidgrain growth. Several examples of suitable processing methods arediscussed further below.

According to at least one embodiment, a nanocomposite material maycomprise a nanoparticle material that is insoluble in water. This may beof particular importance when the intended use for the optical elementincludes exposure to water. In such instances, at least one component ofthe nanocomposite must be insoluble and stable in water. According to afurther embodiment, a nanoparticle material may comprise a firstnanoparticle material that is soluble in water, and a secondnanoparticle that is insoluble in water. In specific instances, thisdescribes nanoparticle materials that are mutually insoluble.

As discussed herein, metal sulfide powders such as ZnS are useful asprecursors for the optical ceramics used in optical elements, includingsensor windows and domes. ZnS and ZnSe are also two examples of LWIRtransmitting materials. However, these materials typically lack therequisite durability to withstand sustained exposure to harshenvironmental conditions. According to one or more embodiments, usingsulfide-based nanoparticle materials, such as ZnS, in combination withother nanoparticle materials, produces a durable nanocomposite materialthat is suitable for harsh field conditions, such as those experiencedduring aerospace and desert operations.

FIG. 2 is a graph showing a phase diagram of a ZnS—Sm₂S₃ (samariumsulfide) system (O. M. Aliev, K. A. Aliev, and S. M. Gradzhiev,“Reactions in ZnS—Sm₂S₃ and Sm₂S₃ Systems,” Russian Journal of InorganicChemistry, 36 p. 1476 (1991)). As shown, the eutectic compositions andtemperatures are 37 and 56 mol. % Sm₂S₃ and 1250° C. and 1320° C.,respectively. ZnSm₂S₄ and ZnSm₄S₇ are formed in this system. ZnSm₂S₄melts congruently at 1395° C. and ZnSm₄S₇ melts incongruently at 1450°C. 1:1 (spinel) and 1:2 compounds are common to many MeS-RE₂S₃ systems,where Me represents a metal sulfide, and RE₂S₃ represents a rare earthsesquisulfide, such as samarium or yttrium. According to certainaspects, it is predicted that ZnS—Y₂S₃ should behave in a similar manneras the ZnS—Sm₂S₃ system shown in FIG. 2, with the two regions ofinterest including the ZnS—Zn(Sm or Y)₂S₄ region and the Zn(Sm orY)₂S₄—Zn(Sm or Y)₄S₇ region. As appreciated, the kinetics of thereactions may influence the phases actually present in the resultingcomposite material.

A list of potential sulfide-based nanocomposite candidate materials thatare capable of forming second phases for the development of sulfur-basednanocomposite materials, and particularly, ZnS-based nanocompositematerial, are shown in Table 1. A full list of potential sulfidematerials and their physical properties are shown in FIG. 3. Therefractive indices shown in FIG. 3 reflect values obtained for the LWIRrange. The refractive index estimates shown in Table 1 and FIG. 3 arethe oxide values plus 0.3-0.5.

TABLE 1 Nanocomposite Candidate Materials Knoop Melting CrystallineRefractive Space Hardness Point Solubility in Material Form Index Group(kg/mm²) (° C.) water CaS Cubic 2.137 Fm-3m 2525 Slight Y₂S₃ Cubic2.16-2.36 (est.) 1925 Soluble MgS Cubic 2.271 Fm-3m >2000 Decomposes ZnSCubic 2.2907 (1 μm) F-34m 128-276 1185 Insoluble (CVD) 2.201 (10 μm)Ga₂S₃ Monoclinic 2.22-2.42 (est.) 1255 Decomposes ZnS Cubic 2.3884 (0.55μm) F-43m 128-276 1185 Insoluble (MS) 2.1710 (12 μm) CaLa₂S₄ ~2.4

Referring to the materials listed in Table 1, calcium sulfide (CaS) isconsidered less desirable of a candidate than the other listedmaterials, since the value for the refractive index is not as close tothat of ZnS as the other materials. For example, yttrium sulfide (Y₂S₃),magnesium sulfide (MgS), gallium sulfide (Ga₂S₃), and calcium lanthanumsulfide (CaLa₂S₄) have refractive indices that that are close in valueto that of ZnS, where Δn is <0.15. Of these materials, both MgS andGa₂S₃ decompose in water, rendering them unsuitable for use innanocomposite coatings and optical elements exposed to water. Of theremaining materials, both Y₂S₃ and CaLa₂S₄ represent good candidates forforming a multi-phase nanocomposite material with ZnS, and in fact, eachof these materials was combined with ZnS into a nanocomposite material,as discussed below in reference to Examples 1 and 2. Other materialslisted in FIG. 3 may provide good candidates as well. For instance,dysprosium sesquisulfide (Dy₂S₃) and gadolinium sesquisulfide (Gd₂S₃)may also be capable of forming second phases for the development ofsulfur-based nanocomposite materials. Other sulfides that may notnecessarily be listed in FIG. 3 but may also be suitable candidatesinclude one or more of the rare earth elements.

The relationship between the grain size and the hardness of the materialis known as the Hall-Petch (HP) relationship, and according to thisrelationship, mechanical strength increases as the grain size of thematerial decreases. The HP relationship holds until the grain size isthe same as the equilibrium distance between dislocations in the grainstructure. In fact, the Hall-Petch relationship has been shown to breakdown for some materials with fine enough grain sizes such that the plotexhibits a departure from the linear relationship and in certaininstances for very fine grain sizes, takes on a negative slope. For manymaterials, this transition from grain size strengthening to grain sizesoftening is called the “inverse” Hall-Petch relationship (I-HP) and isobserved at the critical grain size. This implies that the mechanicalproperties are progressively dominated by the behavior of the grainboundaries in their response to stress as opposed to bulk mechanicalproperties of the grains themselves. For some materials, this“softening” of the material for grain sizes smaller than the criticalvalue has been observed to occur at grain sizes less than 100 nm, andtypically in the region of several to a few tens of nanometers. Thecritical size may be termed as “the point of Hall-Petch departure.”Implicit in the observation of empirical Hall-Petch plots, therefore, isthat the strongest, hardest version of a particular material will bematerials with an average grain size as close as possible to theHall-Petch departure grain size.

FIG. 4 is a graphical representation of the Hall-Petch relationship andplots the hardness of ZnS and a Ga:ZnS nanocomposite as a function ofgrain size. The shaded region in FIG. 4 represents the grain sizes ofmedium-wave infrared radiation (MWIR) in the range of 3.0 microns to 5.0microns (λ=3-5 μm) nanocomposite optical ceramic (NCOC) materials. Thecritical grain size is about 15 nm, which ensures that the HP effect,and not the I-HP effect, will dominate the material's performance.Generally speaking, the target grain size is less than 100 nm, and insome embodiments, the target grain size is about 50 nm. Further, FIG. 4illustrates that it can be expected that the hardness of ZnS may beincreased to values at about 7.5 GPa through grain size refinement usinga nanocomposite approach to fabrication. Ga:Zn nanocomposites are alsopredicted to have hardness values that are twice as high as those ofMgF₂, which results in materials that possess substantially bettererosion resistance. Further, the hardness of ZnS nanocomposite materialshaving grain sizes similar to MWIR NCOC materials, e.g., 80 nm, arepredicted to be up to five times higher than those of multispectral(MS)-ZnS materials. Multispectral ZnS is a form of chemical vapordeposition (CVD) zinc sulfide (otherwise referred to as CVD ZnS) that issubjected to a post-deposition hot isostatic process. This removes zinchydrides from the crystal lattice, normalizes crystal structure, andpurifies the material, thus creating a virtually clear substrate withhigh transmission and minimal scatter from 0.4 microns to 12 microns.

According to some embodiments, the nanocomposite material exhibits aKnoop hardness of greater than 600 kg/mm². According to otherembodiments, the nanocomposite material exhibits a hardness of greaterthan 1000 kg/mm². According to a further embodiment, the nanocompositematerial exhibits a hardness of greater than 1500 kg/mm² (with a 50 gload). The hardness may vary depending on the materials used, thethickness of the resulting material, and the ratio of nanoparticles usedin the nanocomposite. According to a further aspect, the hardness of thenanocomposite material is greater than the hardness of any of thestarting nanoparticle materials that comprise the nanocomposite.

Method of Manufacture—Optical Element

According to some embodiments, a method for producing an optical elementcomprising at least one nanocomposite material is provided. As mentionedabove, the optical element may be any optical element, including awindow, dome, or lens structure, including the dome 102 shown in FIG.1A. ZnS has been proven to be a useful precursor for the opticalceramics used in sensor windows and domes on aircraft, satellites, andmissiles. ZnS windows have traditionally been prepared by hot pressingZnS powders, or in an alternative method, by CVD processes. The CVDprocess may be advantageous since it allows ZnS windows to be fabricatedin larger, flat, or curved geometries which readily conform to the shapeof an aircraft. As will be appreciated, conventional processing oftwo-phase sulfide powders require hot pressing with sintering. Thisresults in excessive grain growth that translates into poor mechanical,optical, and thermal properties. At least one method disclosed hereinand disclosed below overcomes the poor sintering behavior of covalentlybonded sulfide ceramic materials.

According to at least one embodiment, a block flow diagram of an exampleprocess, generally indicated at 500, for producing a two-phasenanocomposite material is shown in FIG. 5. A first nanoparticle material522, which in this instance is Calcium Lanthanum Sulfide (CaLa₂S₄), orCLS, was first prepared into nanoparticle form through a series of stepsindicated at 526-532. Although CLS is used as a specific material inreference to this example process, it is appreciated that one or moreother materials, such as one or more of the materials listed above inreference to Table 1 or FIG. 3, may also be used.

The first nanoparticle material 522, was obtained through commerciallyavailable sources, such as from Lorad Chemical Corp. (St. Petersburg,Fla.). Specifically, CLS powder having a particle size of about 3microns was mixed with reagent grade ethanol 524 to form a slurry withapproximately 8% CLS powder by volume. According to other examples,methanol or isopropyl alcohol may also be used instead of ethanol. Atstep 526, the CLS/ethanol mixture was then placed into a jar mill onrollers (Paul O. Abbe, Bensenville, Ill.) with a zirconium dioxide(ZrO₂) grinding media (3-5 mm diameter) for a time period of from about24 hours to about 96 hours. The milling process is based on impactparticle size reduction and functions to break up agglomerates ofmaterial to produce the nanoparticles. At steps 528 and 530 a settlingand separation process was performed where the nano-sized particlefraction of the CLS was separated from the ethanol. For example, at step528 the CLS/ethanol mixture was allowed to settle and at step 530 thesupernatant (nano-sized CLS particles and ethanol) was removed from theCLS milled slurry. In step 532, the ethanol was removed via anevaporation process and the CLS material was dried and crushed, whichresulted in nanoscale-sized CLS (otherwise referred to as nCLS)particles. Evaporation and drying was performed using a pan dryingprocess in an oven at 110° F. At this point in the process, the majorityof the milled nCLS had a particle size of less than 100 nm, with someparticles having a size of about 61 nm. The dried and crushed nCLSpowder was also passed through a 100 mesh nylon sieve.

At step 538, the first nanoparticle material 536, which in this exampleis CLS, and a second nanoparticle material 534, which in this example isZnS (otherwise referred to as nZnS) were mixed with ethanol 524 in aball or jar mill, such as the mill used above in step 526, to form aslurry with approximately 8% solids by volume, although according toother examples, a higher solids loading may be used to make the processmore efficient. According to this specific example, the milling mediaconsisted of ZrO₂, but other milling media is within the scope of thisdisclosure. For example, low mill wear hard grinding media with highalumina content, such as Al₂O₃, may also be used. The first and secondnanoparticle materials were mixed thoroughly for about 24-96 hours untila uniform mixture was achieved. Different volume ratios of ZnS to CLSwere tested, including 90:10, 80:20, 70:30, and 60:40.

The nZnS material may be obtained through commercially availablesources, such as from Texas Biochemicals, Inc. (College Station, Tex.)that ranged in size from about 15 nm to about 300 nm. In alternativeembodiments, additional nanoparticle materials may also be added to thenanocomposite mixture at step 538. Thus, three or more nanoparticlematerials may be included in the resulting nanocomposite material.

After mixing, the nanocomposite mixture was pan dried in an oven at 110°F. to remove the ethanol, and then crushed and sieved in step 540. Forexample, the dried and crushed mixture was passed through a 100 meshnylon sieve. At step 542, the nanocomposite mixture was poured into adie and isopressed. As will be appreciated, the die may take on any formaccording to the targeted application. For example, the die may form adome or window structure. In this particular example, the powder waspoured into a 12 mm diameter mold and an isostatic pressure of 30,000psi was applied.

In step 544, the nanocomposite was subjected to pressureless sintering(i.e., at ambient pressure, or without the application of mechanicalpressure), which functioned to densify or consolidate the nanocompositematerial. The sintering may also be done using a spark plasma sintering(SPS) device at an elevated mechanically applied pressure. According tothis specific example, sintering was performed at 1100° C. for 6 hours,and resulted in a density that was greater than 97% of theoreticaldensity. Sintering was followed by hot isostatic pressing (HIP) at step546, which in this example occurred at a temperature of 990° C. for 12hours. The process described in FIG. 5 is thus capable of constructingan optical element with a multi-phase LWIR nanocomposite material.

As will be appreciated, the process described above in reference to FIG.5 may be altered or adjusted according to a specific desiredapplication. For example, different temperatures, pressures, and timeperiods may apply, depending on the individual nanoparticle materialsand the optical element.

In accordance with some methods of fabricating optical elements, two ormore nanoparticle materials that comprise the nanocomposite material areuniformly mixed together. For example, referring to FIG. 5, mixing atstep 538 may include uniform mixing. As used herein, the term “uniformlymixed” may refer to equally distributed particles, equally spacedparticles, or both. In general, uniformly mixed means that each of thenanoparticle materials are individually dispersed in a generally uniform(as measured by their relative spaced) manner. Thus, uniform mixing maybe accomplished at the nanoscale level, and in some instances, may beaccomplished at the atomic scale. In a non-limiting example, mixing maybe performed using a rotating mixing chamber.

According to some embodiments, the nanocomposite material includes twoor more nanoparticles materials in equal proportion. For example, 50% ofthe nanocomposite material by volume is a first nanoparticle material,and the other 50% of the nanocomposite coating is a second nanoparticlematerial. According to another embodiment, the nanocomposite materialincludes two or more nanoparticles materials that are in unequalproportions. For example, the first and second nanoparticle materialsmay be present in a 1:2 ratio. The proportion of each material isdependent on the type of materials and the specific application. Forexample, some applications may include mixtures of materials that are ina volume ratio ranging from about 20:80 to about 80:20. In otherapplications, the mixture of materials may be in a volume ratio rangingfrom about 10:90 to about 90:10. In still other applications, themixture of materials may be in a volume ratio ranging from about 5:95 toabout 95:5. A specific example of a nanocomposite material producedusing the process shown in FIG. 5 is discussed below in reference toExample 1.

Method of Manufacture—Coating an Optical Element

According to some aspects, a method for coating at least a portion of asurface of an optical element with one or more nanocomposite materialsis provided. For example, the optical element may include at least onesurface that is at least partially covered by the nanocomposite coating,such as the arrangement shown in FIG. 1B. According to some embodiments,the nanocomposite material may be deposited using a sputteringtechnique. FIG. 6 illustrates an example of a magnetron RF sputteringdevice, generally indicated at 600 (adapted from Joung et al. NanoscaleResearch Letters 2012 7:22) that may be used for such coatingapplications. The sputtering device 600 includes a chamber 666, one ormore targets 670 and 672, an RF power supply 668, and a substrate 660.Each target 670 and 672 includes a source target material species, suchas ZnS and Y₂S₃, where each material species contributes toward themulti-phase structure of the nanocomposite coating material. Thesputtering device 600 featured in FIG. 6 includes two target species,but as will be appreciated, additional target species may be added.During operation, the species from targets 670 and 672 are co-sputteredonto the rotating substrate 660 that is also positioned within thechamber 666 using one or more RF power supplies 668. According to someembodiments, the substrate 660 may be an LWIR transmitting material thatis to be at least partially coated with the nanocomposite coating. Itwill be appreciated that the individual choices for the first and secondnanoparticle materials (i.e., target material species 670 and 672) maydepend on the type of application and the type of optical element thatis being used for a particular application. For example, many opticalelements are constructed from ZnS, and therefore having thenanocomposite coating material include ZnS is advantageous, since therespective refractive indices are identical or nearly identical. Thus,according to some embodiments, the substrate may be ZnS, and one of thetargets 670 or 672 may also be ZnS. The substrate 660 featured in FIG. 6is planar in shape, but other shapes within the scope of this disclosureinclude other optical elements, including domes, windows, and lenses.Sputtering gas 662, such as argon (Ar), enters the chamber 666, which iskept at vacuum using pump 664. As will be appreciated, the compositionalratio of the individual target material species into the resultingnanocomposite coating material may be controlled by through the RF powersource(s) 668. Further, the thickness of the coating may also becontrolled using process parameters, such as the length of processingtime and adjusting the RF power source(s) 668. A specific example of ananocomposite coating material produced using a magnetron RF sputteringdevice shown in FIG. 6 is discussed below in reference to Example 2.

Other methods of coating are also within the scope of this disclosure.For example, ion beam sputtering and CVD techniques may also be used. Asmentioned previously, process parameters, including pressure, may becarefully controlled to limit the grain size of the resultingnanocomposite structure.

Example 1—ZnS-CLS Composite

To construct and test the behavior and physical properties of atwo-phase nanocomposite coating material using a first candidatematerial selected from Table 1 (and FIG. 3) above, ZnS and CLSnanoparticles materials were prepared and combined as discussed above inreference to FIG. 5. Specifically, a mixture of 30% (by volume) CLS wasmixed with ZnS to form a nanocomposite material having a thickness of0.1 mm. A first SEM image, indicated at “A” on the left side of FIG. 7,shows the top surface of the ZnS/CLS nanocomposite material at amagnification of 500×. A second SEM image, indicated at “B,” is alsofeatured in FIG. 7, and shows the top surface at a magnification of2000×. Image “B” also includes a first arrow pointing toward a firstphase of the nanocomposite material. As discussed below, the first phaserepresents the CLS nanoparticle material, and is represented as thewhite needle structures in FIG. 7. Image “B” also includes a secondarrow pointing toward a second phase of the nanocomposite material,which represents the ZnS matrix.

A third SEM image, indicated at “C” is featured on the left side of FIG.8, and shows the top surface of the nanocomposite material at amagnification of 5293×. As mentioned above, the needle-like CLSstructures are labeled as the first phase (i.e., Region 1) and the ZnSmatrix region is labeled as the second phase (i.e., Region 2). The topgraph on the right side of FIG. 8 shows the energy dispersive X-rayspectroscopy (EDS) taken by the SEM for Region 1 representing the firstphase CLS material. Region 1 indicates strong peaks for the Ca, La, andS components of CLS. The bottom graph on the right side of FIG. 8 showsthe SEM/EDS information taken for Region 2 representing the second phaseZnS material, and indicates strong peaks for the Zn and S components ofZnS. The image analysis performed above confirms that the nanocompositematerial includes two distinct composition regions, and that thenanocomposite material contained about 26% by volume CLS needles.

Further analysis was performed on the CLS/ZnS nanocomposite material,and the results are shown in FIGS. 9 and 10 and Table 2 below. FIG. 9 isan x-ray diffraction (XRD) spectrum, and FIG. 10 is a plot of percenttransmission versus wavelength for the single layer of nanocompositematerial. Referring to FIG. 10, it is evident that the nanocompositematerial transmits light in the LWIR range of wavelengths, i.e., 8-12microns.

Table 2 below lists the hardness data for the CLS/ZnS nanocompositematerial, as well as that of other related materials, such as MS ZnS andCVD ZnS. The results indicate that the CLS/ZnS nanocomposite materialexhibits superior hardness compared to MS-ZnS, CVD ZnS, and singlecrystal ZnS. For example, the CLS/ZnS nanocomposite material is about1.4 times harder than MS-ZnS. Thus, the above data indicates that thenanocomposite material improves the mechanical properties of ZnS,without adversely affecting in any significant manner the opticalproperties, such as refractive index and transmittance.

TABLE 2 Hardness Results CLS-ZnS MS-ZnS Single X'tal CVD ZnS HK₂₅ 226160 203 217 (Knoop Hardness) (kg/mm²) Grain Size (μm) 1-10 25-50 — 2-8

Example 2—ZnS—Y₂S₃ Composite

A second test was performed to test the behavior and physical propertiesof another two-phase nanocomposite material using a second candidatematerial selected from Table 1 (and FIG. 3). Specifically, ZnS wasco-sputtered with Y₂S₃ onto an MS-ZnS substrate having a thickness of 3mm using an RF magnetron sputtering device, such as the device featuredin FIG. 6. The process was performed at 100° C. and a pressure of 3mTorr using argon (Ar) as the sputtering gas. Several thicknesses andconcentrations of Y₂S₃ were tested, with the resulting hardness valuesdisplayed below in Table 3.

TABLE 3 Hardness Results Y₂S₃ Knoop Hardness Run Number ThicknessConcentration (kg/mm²) Substrate (MS ZnS) 3 mm — 167 1 42 μm  0% 508 238 μm  7% 593 3 40 μm 16% 645 4 35 μm 20% 665

The results shown in Table 3 indicate that hardness of the nanocompositecoating material increases with increasing yttrium concentration, whichestablishes that there is improved hardness when ZnS is co-sputteredwith Y₂S₃ as a second phase. In fact, the hardness increased by overthree-fold compared to the MS ZnS substrate (see Table 2), and is asmuch as 30% harder than conventional ZnS REP coatings.

Referring to FIG. 11, the percent transmittance as a function ofwavelength for run numbers 1-3 of Table 3 are shown. Also included inFIG. 11 is data from a 40 μm thick layer of ZnS on an MS-ZnS substrate.The results shown in FIG. 11 indicate that the IR transmittance of thickY₂S₃/ZnS films deposited onto a MS-ZnS substrate exhibit little to noreduction in transmittance. This means that materials such as Y₂S₃ maybe used in nanocomposite coating materials to increase hardness anddurability while at the same time preserving the relevant opticalproperties.

According to at least one embodiment, the nanocomposite materialsdiscussed above may further comprise an anti-reflective coating (ARC)layer such as those typically used in optical devices. For instance, thenanocomposite material may further include an oxide that functions as adurable ARC and further aids in protecting the device. In certaininstances, the ARC layer may also function to increase transmittance.

According to certain aspects, the methods and systems described aboveprovide the ability to modify a ZnS based REP coating by forming ananocomposite coating material using two or more compounds of LWIRtransparent nanoparticle materials. The nanocomposite materialsdescribed above are transparent to LWIR radiation and function to hardenLWIR transparent optical elements like ZnS or ZnSe against damage fromhigh speed raindrop and sand particle impact. In certain embodiments,the nanocomposite material may transmit, during use, at least 75% of thelight received. In some embodiments, the nanocomposite may transmit atleast 90%, and in other instances may transmit at least 95% of the lightreceived. According to some embodiments where an ARC layer is used,transmittance may approach 100%.

The combination of relatively high hardness and high degree oftransparency provides commercial value, since the optical elements lastlonger and therefore do not need to be replaced as often. The choice ofusing mixtures of nanoparticle materials that are mutually insoluble ina single nanocomposite material also lends to their increaseddurability. Further, deposition methods, including sputtering, allow forco-deposition of nanocomposites for a large variety of LWIR transparentmaterials.

Having thus described several aspects of at least one example, it is tobe appreciated that various alterations, modifications, and improvementswill readily occur to those skilled in the art. For instance, examplesdisclosed herein may also be used in other contexts. Such alterations,modifications, and improvements are intended to be part of thisdisclosure, and are intended to be within the scope of the examplesdiscussed herein. Accordingly, the foregoing description and drawingsare by way of example only.

What is claimed is:
 1. A method of forming an optical element comprising: combining a plurality of nanoparticles of a first sulfur-based material with a plurality of nanoparticles of a second sulfur-based material such that the plurality of nanoparticles of the first sulfur-based material define a first nanophase, and the plurality of nanoparticles of the second sulfur-based material define a second nanophase; using the combination of the plurality of nanoparticles of the first sulfur-based material and the plurality of nanoparticles of the second sulfur-based material to produce a nanocomposite material having a Knoop hardness value of at least 600 kg/mm² that is greater than a hardness value of either the first or the second sulfur-based nanoparticle materials, wherein the nanocomposite material is at least partially LWIR transmitting, and the first nanophase and the second nanophase are co-dispersed to form interpenetrating networks with one another and each has a grain structure that is distinct from one another; forming the optical element from the nanocomposite material; and coating a surface of the optical element with a first layer of the nanocomposite material having a thickness of less than 50 microns.
 2. The method of claim 1, wherein the plurality of nanoparticles of the first and the second sulfur-based materials are mutually insoluble and at least one of the plurality of nanoparticles of the first and the second sulfur-based material is insoluble in water.
 3. The method of claim 1, wherein combining includes uniformly mixing; and wherein forming the optical element includes isostatically pressing the plurality of nanoparticles of the first and the second sulfur-based materials such that a second layer of nanocomposite material forms the optical element having a thickness of from about 1 mm to about 25 mm.
 4. The method of claim 1, wherein combining and coating include sputtering the plurality of nanoparticles of the first sulfur-based material with the plurality of nanoparticles of the second sulfur-based material such that the first layer of nanocomposite material forms the coating.
 5. The method of claim 1, wherein the plurality of nanoparticles of the first sulfur-based material are zinc sulfide (ZnS), and the plurality of nanoparticles of the second sulfur-based material are calcium lanthanum sulfide (CLS) or yttrium sulfide (Y₂S₃).
 6. The method of claim 1, wherein the volume ratio of the plurality of nanoparticles of the first sulfur-based material to the plurality of nanoparticles of the second sulfur-based material is in a range of about 10:90 to about 90:10.
 7. The method of claim 4, wherein sputtering is performed using a radio frequency (RF) magnetron sputtering device.
 8. The method of claim 7, wherein sputtering is performed at a temperature of about 100° C.
 9. The method of claim 1, wherein the first layer of nanocomposite material has a Knoop hardness of at least 1000 kg/mm².
 10. The method of claim 9, wherein the first layer of nanocomposite material has a Knoop hardness of at least 1500 kg/mm².
 11. The method of claim 1, further comprising depositing an anti-reflective coating onto the first layer of nanocomposite material. 